The most recent statewide surveys documented a rapid decline in the number of Tricolored Blackbirds, from about 400,000 in 2008 to about 250,000 3 years later. Annual monitoring and research into the relationship between reproductive success (RS) and insect abundance has shown a chronic low RS in dozens of colonies studied since 2007 and relatively high RS only in instances when colonies were surrounded by landscapes with unusually high insect abundance. Thus, the recent decline in the numbers of tricolors is believed to be due to chronic low RS (the number of birds recruiting into the population has been much smaller than the number dying) due to insufficient insect abundance. The decline in abundance is seen in the sizes of the largest colonies (Figure 1) from 2005 – 2013, where as recently as 2006 the average of the 5 largest colonies was nearly 72,000 birds, but in 2013 was less than 12,000 birds. And this decline in the sizes of the largest colonies comes at a time when researchers have documented a large number of colonies in new locations (Figure 2), nearly 100 since 2005. All else being equal, such a rapid increase in the number of known colony locations should have resulted in an increase in the population estimate, so the rapid decline in abundance is even more alarming. The next statewide survey, scheduled for April, 2014, will help to quantify the rate of decrease in the numbers of tricolors and provide a current statewide population estimate.
The Los Angeles Times and Sacramento Bee have each featured a story on Tricolored Blackbirds, the Times on June 20th and the Bee on June 21st, 2013. Both stories summarize the results of the work of the many participants in the 2008 and 2011 Statewide Surveys and highlight the continuing rapid decline in abundance of the species throughout California. Due to chronic low reproductive success since 2007 and a variety of other factors, the number of tricolors has been falling rapidly and the 2014 Statewide Survey will be an essential barometer of the species' health.
This publication by U.C. Davis ecologist Bob Meese, which appeared in the journal Western Birds in summer, 2013, summarizes the results of 6 years of field work designed to evaluate the role of insect abundance on tricolored blackbird reproductive success.
A total of 47 colonies was studied and relative insect abundance and reproductive success at each was estimated. In only rare instances was reproductive success estimated to be 1 young/nest or greater, and in each case higher reproductive success was correlated with unusually high insect abundance in foraging habitats surrounding colonies.
Tricolor reproduction appears to be food-limited and few locations in the Central Valley appear to have the abundance of insects necessary to support reproduction by a colonial insectivorous bird species.
The southern California tricolor population, already reduced to less than 6,000 birds (2011 statewide survey), has fared poorly in 2013 due to a series of events at locations that recently supported the region’s largest colonies. In northern Los Angeles County, two sites, Holiday Lake and Fairmont Reservoir, are not supporting tricolor colonies in 2013. Holiday Lake was drained in March, 2013 to allow the Los Angeles County Fire Department to burn the senescent cattails. As of the third week of May, 2013 the basin has not been refilled and remains nearly empty; thus, there is no nesting habitat for tricolors at Holiday Lake this year. Fairmont Reservoir, approximately 10 miles to the southeast of Holiday Lake, is dry and does not provide nesting habitat. Thus, tricolor breeding at both L.A. County locations is prevented by an absence of nesting habitat this year.
Chris Wilson, the Director of Conservation Science at the Santa Lucia Conservancy, is placing radio transmitters on breeding tricolored blackbirds on the Santa Lucia Preserve, a private reserve in Monterey County. His research, which started in May, 2012 has found that tricolors on the preserve forage extensively upon the caterpillars of oak moths (Phryganidia californica) high in oak trees, which may be the first time that arboreal foraging has been documented in the species. His work has also shown that breeding tricolors may suddenly cease breeding and move to a different nearby breeding colony. Chris will continue his research in 2013.
A report by U.C. Davis ecologist Bob Meese in the spring, 2012 issue of California Fish and Game summarizes the effects of cattle egret predation on tricolored blackbird breeding colonies. Cattle egrets, which have only relatively recently expanded their range into California, have since 2006 been severely reducing the reproductive success of tricolor colonies within the foraging range of their rookery in Tulare County. This new report documents the impacts of cattle egret predation from 2006 through 2011 and considers the conservation implications of continued losses of tricolor productivity to cattle egret predation.
Results of Kelly Weintraub's 2011 field season in the Tulare Basin determining the nesting success rates of Tricolored Blackbird colonies in various nesting substrates. This report was prepared for the Fish and Wildlife Service as an annual report.
Methodology, results, and discussion of Tricolored Blackbird nesting success are included.
The results of the 2011 Statewide Survey have been released by Audubon California's Keiller Kyle, the survey coordinator.
The results show a dramatic drop in the number of tricolors statewide, from approximately 395,000 in 2008 to about 259,000 this year, a greater than 33% decrease, this despite an increase in the number of sites visited and an immense effort by 100 survey participants.
The results of this year's survey are summarized in a report from Kyle and Rodd Kelsey, the 2008 Statewide Survey coordinator. Download the report from the portal.
We thank the many volunteer observers for their dedication and hard work in helping to monitor the abundance, distribution, and breeding habitats utilized by the tricolor - California's blackbird.
Information for the Environment (ICE) staff at the University of California, Davis has recently upgraded the Tricolored Blackbird Portal to facilitate access to data, provide greater data input functionality, and to support the 2011 Statewide Survey and all future statewide surveys.
These enhancements include: